Naked Single (Singleton, Sole Candidate)

Whilst solving Sudoku puzzles, it is often the case that a cell can only possibly take a single value, when the contents of the other cells in the same row, column and block are considered. This is when, between them, the row, column and block use eight different digits, leaving only a single digit available for the cell.

For example, in the partial Sudoku puzzle below, the marked cell can only be a 6. All other digits are excluded by the contents of the
other cells in the row, column and block.

So why is this technique called naked single? Simply because if
you use a computer assistant (such as SadMan
Sudoku) that gives you the full and complete candidate listing for all
cells, these cells stand out because they only have a single candidate each.
Contrast this to hidden single.

Here are some Sudoku puzzles that can be solved using only naked single: (What are .sdk files?)